1. Field of the Invention
The present invention relates to developing devices for turning electrostatic images formed on image-bearing members into visible images by adhering developer to the electrostatic images.
2. Description of the Related Art
In image-forming apparatuses employing an electrophotographic recording method such as copying machines, electrostatic images formed on image-bearing members such as photosensitive drums are turned into visible images by adhering developer to the electrostatic images. For such development, developing devices using two-component developer including a toner and a carrier are well known. An example of such developing devices is shown in FIG. 8. FIG. 8 is a schematic cross-sectional view of a developing device according to a known technology.
A developing device 101 using a two-component developer shown in FIG. 8 often includes two horizontally disposed screws, a first carrying screw 105 and a second carrying screw 106, that carry the two-component developer while agitating the developer. The first carrying screw 105 supplies the developer to a developer-bearing member (referred to as a “developing sleeve”) 108, and collects the developer after passing through a developing area A where the developing sleeve 108 faces a photosensitive drum 110.
Moreover, the second carrying screw 106 agitates the developer collected from the developing sleeve 108 and newly supplied developer so as to mix the developers.
A bristle-cutting member 109 regulates the length of bristles of the developer supported on the developing sleeve 108 in a developing container 102.
On the other hand, a demand for small image-forming apparatuses employing the electrophotographic recording method such as copying machines and printers has been increasing in recent years to enable space saving. In particular, a demand for small full-color image-forming apparatuses that use a plurality of developing devices is high.
Therefore, for example, developing devices disclosed in Japanese Patent Laid-Open Nos. 5-333691 and 6-51634 are well known.
First, the developing device disclosed in Japanese Patent Laid-Open No. 5-333691 will be described with reference to FIGS. 9 to 12.
FIG. 9 is a transverse sectional view of a developing device 101 in a direction perpendicular to a direction of axes of a developing sleeve 108 and carrying screws 105 and 106, whereas FIG. 10 is a longitudinal sectional view in the axial direction.
The developing device 101 of a vertical agitating type shown in FIG. 9 is peculiar in that it has two vertically disposed carrying screws 105 and 106 for agitating and carrying developer.
More specifically, the developing device 101 includes a container 102 containing the developer, and includes the developing sleeve 108 disposed at an opening of the container 102, the opening facing a photosensitive drum 110. A developing chamber 103 and an agitating chamber 104 that are separated by a partition 107 are vertically disposed in the container 102 at a side opposite to the side of the opening. The first carrying screw 105 and the second carrying screw 106 are disposed in the developing chamber 103 and the agitating chamber 104, respectively, so as to agitate and carry the developer.
The first carrying screw 105 carries the developer in the developing chamber 103. Moreover, the second carrying screw 106 carries toner, which is newly supplied from a toner inlet disposed upstream of the second carrying screw 106, to the agitating chamber 104, and the developer already existing in the agitating chamber 104, while agitating the toner and the developer so as to homogenize the toner density of the developer.
As described above, the developing device 101 of the vertical agitating type shown in FIG. 9 has the developing chamber 103 and the agitating chamber 104 that are vertically disposed, and thus the occupied space in the horizontal direction is advantageously small. Therefore, a small color-image-forming apparatus of a tandem type having a plurality of developing devices disposed in parallel in the horizontal direction, for example, can also be realized.
Furthermore, the developing device of the vertical agitating type also has the following advantages.
That is, as shown in FIG. 9, the developer in the developing chamber 103 is carried to the developing area A while being supported on the developing sleeve 108, and then used for development. Subsequently, the remaining developer that is not used for the development in the developing area A is collected at the agitating chamber 104 in connection with the rotation of the developing sleeve 108 instead of the developing chamber 103. Therefore, the developing chamber 103 always contains only the developer that has been sufficiently agitated in the agitating chamber 104.
Thus, the developing sleeve 108 is always supplied with the developer having uniform density, and uniform images without unevenness and density difference in a direction parallel to the rotational axis (thrust direction) caused by insufficient agitation can be obtained.
The above-described developing device 101 of the vertical agitating type has advantages in that the developing device is suitable for a reduction in size and the unused developer can be returned to the agitating chamber as described above. However, the following problems exist.
That is, as shown in FIG. 10, the first carrying screw 105 is disposed in the bottom portion of the developing chamber 103 so as to be substantially parallel to the axis of the developing sleeve 108. The screw 105 carries the developer in the developing chamber 103 in one direction along the axis thereof by rotation.
Moreover, the second carrying screw 106 is disposed in the bottom portion of the agitating chamber 104 so as to be substantially parallel to the first carrying screw 105. The screw 106 carries the developer in the agitating chamber 104 in a direction opposite to the carrying direction of the first carrying screw 105.
In this manner, the rotations of the first carrying screw 105 and the second carrying screw 106 circulate the developer between the developing chamber 103 and the agitating chamber 104 via openings 111 and 112 provided at either end of the partition 107.
In this developing device 101, the developing chamber 103 and the agitating chamber 104 are disposed in the vertical direction. Therefore, the developer is moved downward from the developing chamber 103 to the agitating chamber 104, and is moved upward from the agitating chamber 104 to the developing chamber 103 as shown in FIG. 10.
In particular, the developer is delivered from the agitating chamber 104 to the developing chamber 103 such that the developer is pushed upward by the pressure of the developer that is accumulated at the end portion. A circulation route of the developer at this time is shown in FIG. 11.
As shown in FIG. 11, all the developer delivered from the agitating chamber 104 to the developing chamber 103 does not reach the downstream end of the first carrying screw 105 in the developing chamber 103. Some components of the developer are supplied to the developing sleeve 108 along the route, and collected in the agitating chamber 104 after passing through the developing area. Routes of these components are shown by arrows B in FIG. 11.
The supply of the developer to the developing sleeve 108 is performed over approximately the entire width of the developing sleeve 108. Therefore, as shown in FIG. 10, the amount of developer carried by the first carrying screw 105 in the developing chamber 103 tends to be gradually reduced from the upstream end to the downstream end. On the other hand, the amount of developer carried by the second carrying screw 106 in the agitating chamber 104 tends to be gradually increased from the upstream end to the downstream end. That is, the distribution of the developer in the developing device 101 is uneven.
In particular, when the developer is unevenly distributed in the developing chamber 103, the supply of developer to the developing sleeve 108 becomes uneven, and this unevenness causes a difference in density in images in the axial direction of the developing sleeve 108. That is, the density in images in the upstream region of the carrying route in the developing chamber where a sufficient amount of developer is supplied stays constant due to the stable supply of developer to the developing sleeve 108. However, images in the downstream region of the carrying route in the developing chamber where a sufficient amount of developer is not supplied are degraded due to the density unevenness of the images caused by the insufficient supply of the developer to the developing sleeve 108.
FIG. 12 illustrates a developing device having a developing chamber and an agitating chamber as in the developing device 101 shown in FIG. 9 and two developing sleeves. In this type of developing device, it is ideal that the developer supplied from a developing chamber 103 to an upper developing sleeve 108a be transferred from the upper developing sleeve 108a to a lower developing sleeve 108b, and the developer removed from the lower developing sleeve 108b be supplied to an agitating chamber 104. Subsequently, the developer supplied to the developing chamber 103 via a route similar to that described above is supplied again from the developing chamber 103 to the upper developing sleeve 108a. 
However, when the developer is unevenly distributed in the agitating chamber 104, in the developing device 101 shown in FIG. 12, the developer may be supplied from the agitating chamber 104 to the lower developing sleeve 108b in a direction of an arrow C shown in FIG. 12 at a position where the developer surface in the agitating chamber 104 is high, i.e., in the downstream region of the carrying route in the agitating chamber.
Contrary to the ideal flow of the developer, an excessive amount of developer is supplied to the lower developing sleeve 108b at the above-described position when a flow in the direction of the arrow C is generated. This may lead to an unevenness of density in images, and thus lead to degradation of the images.
In order to solve the above-described problem, the carrying capacity of the developer of the first carrying screw 105 and the second carrying screw 106 may be sufficiently increased as compared with the amount of the developer supplied to the developing sleeve 108a such that the unevenness of the developer is relatively reduced as in the known technology disclosed in Japanese Patent Laid-Open No. 5-333691. However, when the amount of developer to be carried is increased, by increasing the rotational speed of the first carrying screw 105 and the second carrying screw 106, stress and torque to the developer are increased. Thus, the amount of developer to be carried cannot be significantly increased.
Moreover, increasing the carrying speed by improving the pitch, shape, or the like of the carrying screws is also ineffective due to the upper limit of the carrying speed when the distribution of the developer is markedly uneven.
The known technology disclosed in Japanese Patent Laid-Open No. 6-51634 provides another solution to the above-described problems. That is, the technology includes a third carrying screw 113 shown in FIG. 9 indicated by alternate long and short dash lines. The third carrying screw 113 is disposed between the developing sleeve 108 and the second carrying screw 106 in the agitating chamber 104 so as to smooth the unevenness of the developer. However, the structure of the developing device becomes complicated in this case, and leads to an increase in cost.